In recent years, advancement of fishing lines has been remarkable and fishing lines of different properties tailored to various types of fishing have been developed. Inter alia, braided yarns and covered yarns having a core-sheath structure composed of two or more kinds of fibers including high strength fibers, such as ultra high molecular weight polyethylene fibers, aramid fibers , PBO fibers , polyarylate fibers and glass fibers, have attracted attention because they have high strength, durability and low rate of elongation suitable for easy and correct perception of a fish strike.
Already known as such a fishing line having a core-sheath structure composed of two or more kinds of fibers are as follows: a fishing line which is a covered yarn comprising a synthetic resin multifilament yarn as a core yarn and a twisted synthetic resin multifilament yarn as a sheath yarn wound around the core yarn, the difference between the angle between the core yarn and the sheath yarn and the twist angle of the twisted yarn being 25° or less thereby achieving an excellent breaking strength and knot strength, low rate of elongation and an excellent abrasion resistance (Patent Literature 1); a fishing line comprising a fluorine multifilament fiber as a core yarn and an ultra high molecular weight polyethylene fiber braided around the core yarn, which sinks below water surface, is not easily affected by wind etc., and has a strong abrasion resistance (Patent Literature 2); a yarn comprising a glass fiber core yarn and two or more sheath yarns made of a fiber other than glass fiber, the sheath yarns being braided around the core yarn, the core yarn and the sheath yarns being integrated with a binder resin, the yarn having a rate of elongation of 5% or less (Patent Literature 3).
However, these conventional core-sheath fishing lines do not have sufficient degree of entwinement or binding between the core part and the sheath part. Therefore, such fishing lines have problems that the core part and the sheath part separate from each other and the core yarn slips off, resulting in so-called nude yarn and that friction between the line guide of a fishing rod etc. and the fishing line causes the sheath part to separate and partially form an unorganized mass, so-called nep.
Meanwhile, a fishing line of which the core part and the sheath part are integrated by means of thermal fusion bonding or a binder also has a problem of hardening of the yarn, and resulting curliness and difficulty in handling.
In addition, a fishing line made of an ultrahigh strength fiber, such as an ultra high molecular weight polyethylene filament, has a relatively small specific gravity, and therefore is easily affected by wind or tide. Furthermore, in fast tidal stream or in a deepwater area, it is difficult to quickly and accurately throw the fishing line into a fishable depth range. In recent years, there is a demand from the market for using a fishing line with a specific gravity most suitable for a particular situation, such as in adverse weather conditions or in an area with rapidly changing tidal streams. In this context, development of a yarn with specific gravity of 1.0 or more, preferably adjustable in the range of 1.0 or more, has been desired.